Group III nitride semiconductors have conventionally been utilized as functional materials for constructing Group III nitride semiconductor light-emitting devices having a pn junction-type structure, such as light-emitting diodes (LEDs) and laser diodes (LDs), which radiate short-wavelength visible light (for example, see Japanese Unexamined Patent Publication No. 2000-332364). For example, for constructing LEDs exhibiting light emission in the near-ultraviolet, blue or green bands, n-type or p-type aluminum gallium nitride (AlXGaYN: 0≦X, Y≦1, X+Y=1) is used to form the clad layer (for example, see Japanese Unexamined Patent Publication No. 2003-229645). Also, gallium indium nitride (GaYInZN: 0≦Y, Z≦1, Y+Z=1) is utilized to form the light-emitting layer (for example, see Japanese Examined Patent Publication No. 55-3834).
In conventional Group III nitride semiconductor light-emitting devices, it is common to provide an n-type or p-type Group III nitride semiconductor layer in conjunction with the light-emitting layer. The emission component is constructed as a hetero-junction structure in order to obtain high emission intensity. For example, for constructing an emission component with a double hetero (DH) junction structure, conventionally, the light-emitting layer is composed of GaYInZN (0≦Y, Z≦1, Y+Z=1) and the n-type or p-type Group III nitride semiconductor layer is bonded as a clad layer or the like (for example, see Akazakai I., “Group III-V Compound Semiconductors”, May 20, 1995, Baifukan, Chap. 13).
A light-emitting layer has also been proposed which employs a quantum well structure for improved emission efficiency. Currently marketed light-emitting devices such as blue LEDs and violet laser diodes employ lattice misfit materials formed by laminating different types of semiconductor layers with different lattice constants. Sapphire is generally used as the substrate for Group III nitride semiconductor light-emitting devices, with a GaN layer generally used as the n-type layer and a GaInN-based quantum well structure as the light-emitting layer.
In such light-emitting devices, the a-axis lattice constant of the light-emitting layer is considered to be roughly equivalent to the a-axis lattice constant of the n-type layer under it (coherent growth). Generally, coherent growth occurs when the film thickness of the light-emitting layer is thinner than the critical film thickness, as in a quantum well structure, and relaxed growth occurs when it is thicker than the critical film thickness.
Also, conventional techniques are known for preventing deterioration in characteristics due to distortion, by specifying a range for the average degree of distortion of the a-axis lattice constant of the GaN layer in a laminated structure comprising an n-type layer, light-emitting layer and p-type layer on a sapphire substrate (for example, see Japanese Unexamined Patent Publication No. 2002-124702).